1. Field of the Invention
The present invention relates to a data multiplexer and a multiplexing method and more particularly, is suitably applied to a data multiplexer of a digital broadcasting system.
2. Description of the Related Art
Various compression-encoding methods are recently proposed as methods of reducing the information quantity of pictures and sounds and Moving Picture Experts Group Phase 2 (MPEG2) is the typical one of the compression-encoding methods.
The MPEG2 method is standardized (e.g. Recommendation H.222.0) by an organization such as International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) and specified in order to compression-encode, multiplex and transmit picture data and sound data.
Actually, the MPEG2 method specifies a data format referred to as a program stream (hereafter, referred to as a PS data format) and a data format referred to as a transport stream (hereafter, referred to as a TS data format) as data formats for multiplexing compression-encoded picture data and sound data.
The PS data format is used to accumulate multiplexed picture data and sound data in a predetermined digital accumulation medium, and the TS data format is used to transmit multiplexed picture data and sound data. Moreover, streams having the PS data format and TS data format can be mutually converted with a variable-length Packetized Elementary Stream (PES) packet as a basic element.
In the case of the PS data format, for example, it is specified that compression-encoded picture data and sound data are respectively formed into a PES packet every predetermined unit (e.g. every picture), to form a PS packet string (so-called, a program stream) by time-dividing and multiplexing each of the resultant PES packets.
FIG. 1 shows a transmitter 100 of a digital broadcasting system. The transmitter 100 supplies picture data D1A and D1B corresponding to respective television broadcast programs to corresponding encoders 101A and 101B from data output sections (not illustrated) corresponding to plural channels, compression-encodes the picture data D1A and D1B in accordance with the MPEG2 method in the corresponding encoders 101A and 101B, and successively forms encoded data thus obtained into a PES packet every picture, in order to form and supply encoded data streams D31A and D31B to a multiplexer 102.
To multiplex the encoded data streams D31A and D31B supplied from the respective encoders 101A and 101B, the multiplexer 102 stores and temporarily accumulates plural PES packets included in the encoded data streams D31A and D31B in corresponding buffers 103A and 103B in arriving order and then, reads out and transmits the PES packets from the buffers to a multiplexing section 104 in accordance with the first-in first-out method.
In this case, the buffers 103A and 103B respectively supply the amount of data accumulated in the PES packets to a multiplexing control section 105 as data accumulation signals S2A and S2B and thereby, the multiplexing control section 105 respectively detects the amount of data accumulated in the buffers 103A and 103B, based on the data accumulation signals S2A and S2B.
The multiplexing control section 105 transmits buffer output control signals S3A and S3B to the respective buffers 103A and 103B based on the detection result before the amount of data exceed the maximum data accumulation capacities of the buffers 103A and 103B. Thereby, the section 105 can read out the PES packets from the buffers 103A and 103B in accordance with the first-in first-out method and thus, it is possible to prevent the buffers 103A and 103B from being broken down due to excessive data accumulation.
Then, the multiplexing section 104 receives the encoded data streams D31A and D31B from the respective buffers 103A and 103B and moreover, receives a multiplexing control signal S4 from the multiplexing control section 105 and then, successively forms plural PES packets included in the respective encoded data streams D31A and D31B into a packet (hereafter, referred to as transport stream xe2x80x9cTSxe2x80x9d packet) every predetermined unit (e.g. every 188-byte data) and thereafter, forms a transport stream (TS stream) comprising one TS packet string D32 by time-dividing and multiplexing the resultant TS packets.
Then, the TS stream D32 is modulated by a not-illustrated modulator in accordance with a predetermined method and the resultant transmission signal is transmitted toward a broadcasting satellite (not illustrated) through an antenna. In this way, the transmitter 100 can broadcast television programs for plural channels through the broadcasting satellite.
In the case of the transmitter 100, the encoders 101A and 101B supply the respective compression-encoded picture data (encoded data streams D31A and D31B) to a multiplexer at a fixed bit rate. In this case, when patterns of input picture data D1A and D1B are flat, the amount of encoded data successively generated by the encoders 101A and 101B are reduced due to the fact that the amount of picture information of the picture data D1A and D1B are extremely small.
Therefore, when the encoders 101A and 101B form successively generated encoded data into a PES packet every picture, they keep the transmission rates of the encoded data streams D31A and D31B comprising the PES packet strings constant by adding dummy data originally unnecessary for encoding (hereafter, referred to as invalid data) to valid data in bytes.
The transmission rates of significant information in the encoded data streams D31A and D31B have a problem of being reduced by a value equivalent to added invalid data because the encoders 101A and 101B respectively output encoded data streams D31A and D31B to which invalid data is added.
In view of the foregoing, an object of this invention is to provide a data multiplexer in which multiplexing efficiency can be improved by effectively using the data transmission rate of a transmission system.
The foregoing object and other objects of the invention have been achieved by the provision of a data multiplexer which detects and extracts invalid data assigned to an inputted data stream and multiplexes and outputs the data stream from which the invalid data is extracted. Thereby, it is possible to improve multiplexing efficiency for invalid data.
Moreover, by detecting the amount of extracted invalid data and multiplexing a predetermined data stream onto the data stream from which invalid data is extracted in accordance with the amount of detected invalid data, it is possible to improve multiplexing efficiency by a value equivalent to the multiplexed predetermined data stream instead of the invalid data.